Scientists have found the most Earth-like planet outside our solar system so far. Unfortunately it's searingly hot, covered in lava and orbits its Sun so closely that a year lasts just 8.5 hours.
But still - it's progress.
Orbiting a star 700 light years from Earth, Kepler 78b is the smallest exoplanet whose mass and size are known. It is just 1.2 times bigger than Earth, 1.7 times more massive, and has an almost identical density.
Scientists believe that, like the Earth, Kepler 78b mostly consists of rock and iron. There the similarities end, however. The planet hugs its parent star so closely that nothing could live on its scorching surface.
"It's Earth-like in the sense that it's about the same size and mass, but of course it's extremely unlike the Earth in that it's at least 2,000 degrees hotter," said Dr Josh Winn, from the Massachusetts Institute of Technology, US, who took part in the research published in the journal Nature.
"It's a step along the way of studying truly Earth-like planets."
Dr Chris Watson from Queen's University in Belfast, whose team also studied the planet, said: "Kepler-78 is a scorching lava world that, put simply, shouldn't exist. Its close proximity to its star, and how it got there, is still a mystery. What we do know is that it won't exist forever. Gravitational tides will slowly disrupt Kepler-78, drawing it closer to its star and eventually ripping it apart."
Artists' Conceptions Of Extrasolar Planets
NASA's Kepler Mission Discovers Planet(01 of05)
Open Image ModalIn this handout illustration made available on December 5, 2011 by NASA, the Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star is digitally illustrated. For the first time NASA's Kepler mission has confirmed a planet to orbit in a star's habitable zone; the region around a star, where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth, making it the smallest yet found to orbit in the middle of the habit. Clouds could exist in this earth's atmosphere, as the artist's interpretive illustration depicts. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images) (credit:Getty)
NASA's Kepler Mission Discovers Planet(02 of05)
Open Image Modal In this handout illustration made available on December 5, 2011 by NASA, a diagram compares our own solar system to Kepler-22, a star system containing the first 'habitable zone' planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth. The diagram displays an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star and is about 2.4 times the size of Earth. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images) (credit:Getty)
Extrasolar Planet HD 209458 b, Osiris(03 of05)
Open Image ModalArtist's conception released by NASA of extrasolar planet HD 209458 b, also known as Osiris, orbiting its star in the constellation Pegasus, some 150 light years from Earth's solar system. Scientists have used an infrared spectrum -- the first ever obtained for an extrasolar planet -- to analyze Osiris' atmosphere, which is said to contain dust but no water. The planet's surface temperature is more than 700 Celsius (1330 Fahrenheit).' (credit:Getty)
Planet & Its Parent Star(04 of05)
Open Image ModalPicture released 04 October 2006 by the European Space Agency shows an artist's impression of a Jupiter-sized planet passing in front of its parent star. Such events are called transits. When the planet transits the star, the star's apparent brightness drops by a few percent for a short period. Through this technique, astronomers can use the Hubble Space Telescope to search for planets across the galaxy by measuring periodic changes in a star's luminosity. The first class of exoplanets found by this technique are the so-called 'hot Jupiters,' which are so close to their stars they complete an orbit within days, or even hours. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM (credit:Getty)
Hot Jupiter(05 of05)
Open Image ModalPicture released 04 October 2006 by the European Space Agency shows an artist's impression of a unique type of exoplanet discovered with the Hubble Space Telescope. This image presents a purely speculative view of what such a 'hot Jupiter' (word dedicated to planets so close to their stars with such short orbital periods) might look like. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM (credit:Getty)
Kepler 78b's orbital period and size was previously determined by analysing the amount of light blocked as the planet passed in front of its star.
Because the planet orbits so close to its star, it was also possible to measure gravitational effects that gave away its mass.
American and European teams of astronomers examined data from observatories in Hawaii and the Canary Islands to analyse the star's "wobble" caused by the gravitational tug of Kepler 78b.
A handful of exoplanets the size or mass of the Earth have been discovered, but Kepler 78b is the first for which both values have been measured.
"When you have both the size and the mass of an object, you can calculate its density, and thereby determine its composition," said Dr Andrew Howard, from the University of Hawaii.
The planet was first identified by the American space agency Nasa's Kepler space telescope, which looked for planets crossing in front of 150,000 stars.
"This discovery is a fine example of the great diversity of the exoplanets that we are uncovering, thanks to the tremendous progress in astronomy technology and techniques. Nature seems to like conjuring up planets in the least expected places. Just five years ago this work would have been impossible. As we probe deeper and deeper we are finding that science fact in stranger than science fiction - Kepler-78b certainly fits that bill."